Generally, in automatic transmissions that are applied to a vehicle, a shift control device controls hydraulic pressure through control of a plurality of solenoid valves based on various conditions such as a vehicle speed and a throttle opening ratio. When the driver manipulates the shift lever to a desired shift range, a port conversion in a manual valve occurs and various operating elements of the gear shift mechanism are selectively operated according to the duty control of the solenoid valve using hydraulic pressure supplied from an oil pump. A gear shift is thereby performed.
Because vehicles having such automatic transmissions may be exposed to various driving conditions, companies invest in extensive research for improving performance of the engine and automatic transmission. In particular, in the case of the automatic transmission, research directed to shift control methods for controlling automatic transmissions are widely performed. In one exemplary method, if it is determined that a kickdown shift is performed as a result of a driver's abrupt depression of an acceleration pedal, a transmission control unit controls a line pressure to be a maximum value. Then, if it is determined that a predetermined time has elapsed, the transmission control unit generates a shift signal and performs a shift control. In such a conventional shift control, if a shift signal is generated in a state that each clutch is supplied with normal line pressure, the line pressure is controlled to rise to its maximum value, and a shift is performed after a predetermined time has lapsed. After the shift has been started, line pressure is supplied as an engaging pressure according to 100% of a line pressure duty.
In the above-stated conventional control, the line pressure is preferably determined by multiplying a highest clutch engaging pressure of the clutch engaging pressures for all shift ratios by a safety factor. However, in such a conventional shift control, if a shift is performed, the line pressure is controlled to its maximum value, and the shift signal is generated after the predetermined time has lapsed. Accordingly, as shown in a region A of FIG. 4, a release pressure increases for a moment and then decreases, so that a shift delay time to an actual shift point becomes long. Thus, a shift responsiveness and a shift feeling deteriorate.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.